1. Field of Invention
This invention relates to control systems and, more particularly, to control systems for controlling fluid flows and pressures by controlling flow through a valve.
2. Description of the Prior Art
U.S. Pat. Re. No. 29,383 (Gallatin et al) discloses a system which uses a pair of manif digitally graduated valves for controlling fluid flow. The manifolds provide output fluid that is measured by a transducer system, and calculated pressure readings from the manifolds are directed to a control circuit that converts analog pressure signals to digital values for operating valves.
U.S. Pat. No. 3,055,389 (Brunner) discloses a sensing system in which fluid flow sensed through a tube by directing fluid pressure to either side of the piston. Motion of the piston is transferred to a variable resistor that in turn controls the motor. The motor is in turn used to control an adjusting element in the balanced bridge circuit, and as a bridge is moved to an unbalanced situation, another motor is actuated to control a valve to vary the fluid flow through the valve.
U.S. Pat. No. 3,757,808 (Peterson et al) discloses a fluid control system in which there are two sensors in a fluid duct. One sensor measures fluid temperature, and the sensor is energized so that its temperature is different from that of the fluid. The differential sensors is indicative from the fluid flow in the duct. A valve is actuated in response to the differential output and to various predetermined parameters for controlling the fluid flow in the duct.
U.S. Pat. No. 3,769,998 (Avant) discloses a valve system in which fluid pressure on each side of the control valve is sensed. Fluid flow is controlled by a plurality of valve elements in response to the measured fluid pressure and predetermined parameters.
U.S. Pat. No. 3,930,518 (Fuller et al) discloses a fluid control system using a pair of sensors connected to a differential pressure switch. The output of the differential switch is used to control a valve.
U.S. Pat. No. 4,000,754 (Risk) discloses a control system which operates automatically to response to fluid pressure in a pair of chambers. Fluid pressure is compared by a regulator and as fluid pressure across a valve changes, a feed back fluid flow adjust the valve to maintain a desired pressure.
U.S. Pat. No. 4,026,321 (Kohoe et al) discloses a differential pressure sensor used across a valve for controlling air flow through a duct. Operation of the valve is responsive to, or is a function of, the pressure sensed by the differential pressure sensor.
U.S. Pat. No. 4,253,480 (Kessel et al) discloses a pressure regulator system in which a comparator compares pressure in a tank to a desired to command pressure that is the input of one leg of the comparator. The output of the comparator controls adjustment devices or valves to a regulator.
U.S. Pat. No. 4,277,832 (Wong) discloses a fluid flow system in which a differential fluid pressure detector is placed across a valve.
Each of the patents discussed above utilizes some type of pressure sensing or flow sensing for controlling fluid flow through a valve. The valve is adjusted in response to the sensed pressure or flow.
The apparatus of the present invention utilizes sensors, but compensates for the change in gain of the valve in flow or pressure as a function of the position of the valve. In this manner, less of a pressure drop across the valve is required, and accordingly better control can be achieved over a relatively wide range of operations. This, in turn, allows the use of a larger valve and a smaller pressure drop and this in turn can result in substantial energy savings while still achieving the desired flow rates or pressures.